Dense multidimensional arrays
There are several ways of declaring multidimensional arrays in D.
Multidimensional arrays and ranges
D standard library provides multidimensional shell over arrays and ranges. It is located in std.experimental.ndslice
import std.experimental.ndslice; auto slice = new int [5 * 6 * 7].sliced(5, 6, 7); assert(slice.length == 5); assert(slice.elementsCount == 5 * 6 * 7); static assert(is(typeof(slice) == Slice!(3, int*)));
The simplest way is to use an array of arrays:
int matrix = [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ]; assert(matrix == 1); assert(matrix == 5);
This creates a so-called jagged array, because each element of the outer array can have different lengths:
int matrix = [ [ 1, 2, 3 ], [ 4, 5, 6, 7, 8 ], // this is valid [ 9, 10, 11 ] ];
However, this approach is not so memory-efficient, because the outer array is a separate block of memory containing references to the inner arrays. Array lookups require multiple indirections, so there is a slight performance hit.
Note that with the "jagged" array scheme, the "2nd dimensions" arrays may either all be allocated individually, or simply be slices of a single very big 1D array. Both schemes are valid.
A dynamic rectangular jagged array may be dynamically allocated at once using the multi-dim allocation syntax:
//Allocates a dynamic array containing // 2 dynamic arrays containing // 5 ints int matrix = new int(5, 2);
Note that in this example, the dimensions don't need to be known at compile time. Also note that this works for any amount of dimensions.
D recognizes the inefficiency of jagged arrays, so when all the dimensions of the array are known at compile-time, the array is automatically implemented as a dense array: the elements are packed together into a single memory block, and array access requires only a single indexed lookup:
// This is a dense array int matrix = [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ] ];
Dense arrays are fast and memory-efficient. But it requires that all array dimensions be known at compile-time, that is, it must be a static array. But what about dynamic arrays?
Dense dynamic arrays
There is a way to make multidimensional dynamic arrays dense, if only the last dimension needs to be variable, or if the array is just too big to fit on stack:
enum columns = 100; int rows = 100; double[columns] gridInfo = new double[columns](rows);
This creates a multidimensional dynamic array with dense storage: all the array elements are contiguous in memory.
The idiom for creating dense multidimensional dynamic arrays was first posted to the D newsgroup by User:Monarchdodra.